1. Field of the Invention
The present invention relates to an ink jet printer.
2. Description of the Related Art
Line-type ink jet printers are provided with an endless belt for delivering a printing sheet, and with a print head for discharging ink. The endless belt has width, and the print head is provided with a plurality of ink jet nozzles. The plurality of nozzles are aligned in a widthwise direction of the endless belt. Below, a direction in which the endless belt moves will be termed a longitudinal direction, and a direction orthogonal to that direction (the widthwise direction of the endless belt) will be termed a widthwise direction.
By selecting which nozzle will discharge ink, it is possible to determine the position, in the widthwise direction of the printing sheet, of a point where the ink discharged from the nozzle will adhere. By adjusting the time at which the ink is discharged from the nozzle, it is possible to determine the position, in the longitudinal direction of the printing sheet, of a point where the ink discharged from the nozzle will adhere. By selecting which nozzle will discharge ink, and by adjusting the time at which the ink is discharged from that nozzle, it is possible to determine the position, in the widthwise and longitudinal directions of the printing sheet, of a point where the ink discharged from the nozzle will adhere. A desired pattern can be printed onto the printing sheet by controlling the position, in the widthwise and longitudinal directions of the printing sheet, of the points where the ink will adhere to the printing sheet.
In order to determine the position, in the longitudinal direction of the printing sheet, of the point where the ink adheres by means of adjusting the time at which the ink is discharged from the nozzle, it is necessary to adjust a moving speed of the endless belt that delivers the printing sheet so that this speed is a predetermined speed. If the actual moving speed of the endless belt is slower than the predetermined speed, printing will be performed further towards an upper edge of the printing sheet than desired. Alternatively, the longitudinal length of a pattern that is actually printed will be shorter than desired. If the actual moving speed of the endless belt is faster than the predetermined speed, printing will be performed further towards a lower edge of the printing sheet than desired. Alternatively, the longitudinal length of a pattern that is actually printed will be longer than desired.
The endless belt is usually wound across a pair of driving rollers. In this case, it is easy for the moving speed of the endless belt to vary due to slippage between the endless belt and the driving rollers, variation in the dimensions of an outer circumference of the driving rollers, variation in the position in which the driving rollers are disposed, variations in rotation speed of the driving rollers, etc. The moving speed of the endless belt may also vary due to the weight of the printing sheet being fed by this endless belt. When the actual moving speed of the endless belt differs from the predetermined speed, there is a discrepancy in the position, in the longitudinal direction of the printing sheet, of the points at which the ink adheres. There is thus the problem that printing quality deteriorates.
Techniques have been developed whereby, even if the moving speed of the endless belt varies, printing quality will not deteriorate. One example thereof is taught in Japanese Laid-Open Patent Application Publication No. 2004-17505. With an ink jet printer taught in that reference, the actual moving speed of an endless belt is detected. The time at which ink is discharged from nozzles is then adjusted on the basis of the detected moving speed. With this method, the points at which the ink adheres do not deviate from the desired position in the longitudinal direction of the printing sheet even if the moving speed of the endless belt differs from the predetermined speed.